sensors Article

Development of a Novel Transparent Flexible Capacitive Micromachined Ultrasonic Transducer Da-Chen Pang * and Cheng-Min Chang Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Jian Gong Rd., Sanmin Dist., Kaohsiung 80778, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-909-129-294 Received: 27 April 2017; Accepted: 15 June 2017; Published: 20 June 2017

Abstract: This paper presents the world’s first transparent flexible capacitive micromachined ultrasonic transducer (CMUT) that was fabricated through a roll-lamination technique. This polymer-based CMUT has advantages of transparency, flexibility, and non-contacting detection which provide unique functions in display panel applications. Comprising an indium tin oxide-polyethylene terephthalate (ITO-PET) substrate, SU-8 sidewall and vibrating membranes, and silver nanowire transparent electrode, the transducer has visible-light transmittance exceeding 80% and can operate on curved surfaces with a 40 mm radius of curvature. Unlike the traditional silicon-based high temperature process, the CMUT can be fabricated on a flexible substrate at a temperature below 100 ◦ C to reduce residual stress introduced at high temperature. The CMUT on the curved surfaces can detect a flat target and finger at distances up to 50 mm and 40 mm, respectively. The transparent flexible CMUT provides a better human-machine interface than existing touch panels because it can be integrated with a display panel for non-contacting control in a health conscious environment and the flexible feature is critical for curved display and wearable electronics. Keywords: capacitive micromachined ultrasonic transducer; flexible transducer; silver nanowire transparent electrode; SU-8; human-machine interface

1. Introduction In 1989, Hohm and Hess [1] presented the first capacitive ultrasonic transducer. The earlier transducers were fabricated using anisotropic etching of silicon backplates [1–3]. In 1994, Haller and Khuri-Yakub [4] developed a capacitive micromachined ultrasonic transducer (CMUT) using sacrificial-layer technology. The Khuri-Yakub group at Stanford University had also proposed fabrication improvements and CMUT applications in the literature [5–9]. In 2003, Huang et al. [10] fabricated a CMUT using wafer bonding technology to ensure fewer process steps and better product quality compared with the sacrificial-layer processes. The wafer bonding technology including anodic bonding, fusion bonding, and adhesive bonding has been extensively applied in the fabrication of CMUTs [11–18]. The opaqueness of silicon wafers to visible light causes non-transparent properties in silicon-based CMUTs. Since infrared light can pass through silicon, Chen et al. [19] presented an infrared-transparent CMUT for photoacoustic imaging in 2012. In 2006, Chang et al. [20] pioneered a polymer-based CMUT using sacrificial-layer techniques and later fabricated on a polymer substrate so the CMUT was flexible [21]. The CMUTs on an ultrathin silicon wafer also feature bending characteristics [22]. In 2008, Zhuang et al. [23] etched trenches into silicon wafers and filled them with polydimethylsiloxane (PDMS) to fabricate flexible CMUT arrays. In 2008, Abgrall et al. [24] applied the lamination technique for minimizing residual stress to fabricate SU-8 bonding structures at low pressure and temperature. In 2012, Shi et al. [25] used Polydimethylsiloxane (PDMS) and bonding technology to fabricate a stretchable CMUT but the CMUT

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was not transparent due to the metal electrodes. In 2015, Li et al. [26] fabricated a polymer-based CMUT using photo benzocyclobutene and bonding technology the membrane wasused madea of silicon of silicon nitride and the process temperature was up to 250 °C.but In 2015, Bui et al. [27] polymer◦ nitride the to process temperature was up to 250 2015, Bui et al. [27] used above a polymer-based based and CMUT measure surface roughness. All C. theInpolymer-based CMUTs were nonCMUT to measure surface roughness. All the polymer-based CMUTs above were non-transparent. transparent. Sensors 2017, 17, 1443 2 of 16 The The purpose purpose of of this this research research isisto todevelop developaanovel novelfabrication fabricationprocess process for foraatransparent transparentflexible flexible CMUT so the CMUT can be applied in a display panel for finger hovering that provides a of silicon nitride and the process temperature was up to 250 °C. In 2015, Bui et al. [27] used a polymerCMUT so the CMUT can be applied in a display panel for finger hovering that provides a more more based human-machine CMUT to measure surfacethan roughness. All the polymer-based CMUTs above werefabrication nonadvanced interface the touch panel. roll-lamination advanced human-machine interface than theexisting existing touch panel. A A new new roll-lamination fabrication transparent. technique technique is is proposed proposed for for the the mass massproduction production of ofCMUTs CMUTsatatlow lowtemperature. temperature.The Theroll-lamination roll-lamination The purposethe of this research is tointroduced develop a novel fabrication processiffor a transparent flexible used. method minimizes residual stress at high temperature a bonding process method minimizes the residual stress introduced at high temperature if a bonding process is is used. CMUT so the CMUT can be applied in a display panel for finger hovering that provides a more This method is also simpler than the sacrificial-layer technique.technique. Three different transparent Thisfabrication fabrication method is also simpler than the sacrificial-layer Three different advanced human-machine interface than the existing touch panel. A new roll-lamination fabrication electrodes, indium tin oxide (ITO), zinc oxide (AZO), silver nanowire, are transparent tin aluminum-doped oxide (ITO), aluminum-doped zinc and oxide (AZO), and silver techniqueelectrodes, is proposedindium for the mass production of CMUTs at low temperature. The roll-lamination fabricated and tested in our CMUTs. The transparent electrodes on the vibrating membranes must nanowire, fabricated and tested our CMUTs. The transparent electrodes on the vibrating methodare minimizes the residual stressinintroduced at high temperature if a bonding process is used. survive under term operation. The term performance characteristics of the Thisultrasonic fabrication method ultrasonic is also long simpler than the sacrificial-layer technique. Three membranes must vibration survive vibration under long operation. The different performance transparent flexible are tested flat and are curved surfaces. The proposed CMUT cansilver be easily transparent indium tin on oxide (ITO), aluminum-doped zinccurved oxide surfaces. (AZO), and characteristics ofelectrodes, theCMUT transparent flexible CMUT tested on flat and The proposed integrated with display panels and lighting systems for non-contacting sensing and control in the nanowire, are fabricated and tested in our CMUTs. The transparent electrodes on the vibrating CMUT can be easily integrated with display panels and lighting systems for non-contacting sensing membranes must survive ultrasonic vibration under long term operation. The performance future. Figure shows theFigure research progress our group over the years. and control in1the future. 1 shows theofresearch progress of our group over the years. characteristics of the transparent flexible CMUT are tested on flat and curved surfaces. The proposed CMUT can be easily integrated with display panels and lighting systems for non-contacting sensing and control in the future. Figure 1 shows the research progress of our group over the years.

Figure 1. 1. CMUT CMUT research researchresults resultsfrom fromour ourgroup. group. Figure Figure 1. CMUT research results from our group.

2. CMUT CMUT Design Design 2. 2. CMUT Design Theproposed proposedtransparent transparentCMUT CMUThas hasaasurface surfacearea areaofof33mm mm×× 33 mm and comprises 416 The mm and comprises 416 hexagonhexagonThe proposed transparent CMUT has a surface area of 3 mm × 3 mm and comprises 416 hexagoninscribed vibrating membranes with a diameter of 140 µm, as illustrated in Figure 2. The ITO-PET inscribed vibrating membranes of 140 140µm, µm,asasillustrated illustrated in Figure 2. The ITO-PET inscribed vibrating membraneswith withaadiameter diameter of in Figure 2. The ITO-PET substrate was 125 µm in thickness, the sidewall was 10 µm in width, the cavity was 2 µm in depth, substrate waswas 125125 µmµm in in thickness, was10 10µm µmininwidth, width, cavity µm in depth, substrate thickness,the thesidewall sidewall was thethe cavity waswas 2 µm2 in depth, and the vibrating membranes, which sandwiched a 0.2 µm thick silver nanowire (SNW) transparent and the membranes, which 0.2µm µmthick thick silver nanowire (SNW) transparent andvibrating the vibrating membranes, whichsandwiched sandwiched aa 0.2 silver nanowire (SNW) transparent electrode, were µm inthickness thickness as shown in Figure Figure3.3. 3.The The design dimensions are listed in 1. electrode, were 5 µm in thicknessas asshown shown in in design dimensions are listed in Table 1. electrode, were 55 µm in Figure The design dimensions are listed in Table Table 1. A total of 72 CMUTs with an overall size of 8 cm × 6 cm were fabricated on a 4-inch silicon wafer. A total 72 CMUTs with overall sizeofof88cm cm × × 66cm on on a 4-inch silicon wafer. A total of 72ofCMUTs with an an overall size cmwere werefabricated fabricated a 4-inch silicon wafer.

Figure 2. Schematic Diagramofoffinger finger hovering hovering using capacitive micromachined Figure 2. Schematic Diagram usingaatransparent transparent capacitive micromachined ultrasonic transducer (CMUT). Figure 2. Schematic Diagram of finger hovering using a transparent capacitive micromachined ultrasonic transducer (CMUT). ultrasonic transducer (CMUT).

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Figure 3. Cross-sectional view of a transparent CMUT. Figure 3. Cross-sectional view of a transparent CMUT. Table 1. Transparent CMUT dimensions. Table 1. Transparent CMUT dimensions. Membrane diameter diameter A µmµm Membrane thickness D D 55µm Membrane A 140140 Membrane thickness µm Silver nanowire (SNW) B 160 µm SNW electrode thickness E 0.2 µm Silver nanowire (SNW) electrode electrode diameter B 160 µm SNW electrode thickness E 0.2 µm diameter Sidewall height F 2 µm Sidewall width C 10 µm Sidewall height F 2 µm Polyethylene Sidewall width C 10 µm Indium Tin Oxide (ITO) terephthalate G 0.2 µm H 125 µm electrode thickness (PET) thickness Indium Tin Oxide (ITO) electrode Polyethylene terephthalate G 0.2 µm H 125 µm thickness (PET) thickness 3. Fabrication 3. Fabrication The fabrication of the transparent flexible CMUT was built on a polymer-based CMUT using sacrificial-layer techniques developed earlier. polymer-based applied a PET substrate The fabrication of the transparent flexibleThe CMUT was built onCMUT a polymer-based CMUT using and SU-8 structure and membrane. It was notThe transparent because platinum andagold usedand for sacrificial-layer techniques developed earlier. polymer-based CMUT applied PETwere substrate the electrodes. The membrane. sacrificial-layer fabrication procedure of the polymer-based CMUT, depicted in SU-8 structure and It was not transparent because platinum and gold were used for the Figure 4, is described as follows: fabrication procedure of the polymer-based CMUT, depicted in electrodes. The sacrificial-layer Figure 4, is described as follows: 1. Paste a PET flexible substrate onto a silicon wafer and sputter a 0.3 µm thick platinum electrode. 1. Paste a PET flexible onto a silicon a 0.3 µmfollowed thick platinum 2. Pattern a 2 µm thicksubstrate AZ4620 photoresist to wafer protectand thesputter sidewall area, by soft electrode. baking at ◦ 2. Pattern a 2 µm thick AZ4620 photoresist to protect the sidewall area, followed by soft baking at 95 C for 2 min. °C for 2 min. 3. 95 Electroform 2 µm thick copper as a sacrificial layer. Remove the AZ4620 photoresist. 3. Electroform 2 µm thick copper as sacrificial layer. Remove the membrane. AZ4620 photoresist. 4. Pattern a SU-8 2002 photoresist to aform a sidewall and vibrating Perform a soft bake 4. Pattern a SU-8 2002 photoresist to form a sidewall and vibrating membrane. Perform soft95 bake ◦ ◦ ◦ ◦C at 65 C for 4 min and 95 C for 4 min, and then post exposure bake at 65 C for 2 minaand at 65 °C for 4 min and 95 °C for 4 min, and then post exposure bake at 65 °C for 2 min and 95 °C for 3 min. for 3 min. 5. Develop the SU-8 2002 photoresist to yield etching holes. Perform a hard bake at 95 ◦ C for 5 min. 5. Develop the SU-8 2002 photoresist to yield etching holes. Perform a hard bake at 95 °C for 5 min. 6. Deposit 0.3 µm thick gold to yield the top electrode layer. 6. Deposit 0.3 µm thick gold to yield the top electrode layer. 7. Pattern-etch the top electrode using the AZ4620 photoresist and potassium iodine. 7. Pattern-etch the top electrode using the AZ4620 photoresist and potassium iodine. 8. Remove the copper sacrificial layer to release the vibrating membranes and cavities. 8. Remove the copper sacrificial layer to release the vibrating membranes and cavities. 9. Remove the the silicon silicon wafer wafer to to complete complete the the CMUT CMUT fabrication. fabrication. 9. Remove

This This research research developed developed aa new new roll-lamination roll-lamination fabrication fabrication procedure procedure for for the the transparent transparent flexible flexible CMUT CMUT without without time time and and cost cost for for the the electroforming electroforming process process compared compared with with the the sacrificial-layer sacrificial-layer technique. TheCMUT CMUTexhibited exhibitedtransparency transparencyand and flexibility employing ITO-PET substrate, technique. The flexibility byby employing an an ITO-PET substrate, SU SU 8 structure membrane, silver nanowire electrode. fabrication method applied 8 structure andand membrane, andand silver nanowire electrode. TheThe newnew fabrication method applied the ◦ C. the roll-lamination technique coating process theprocess processtemperatures temperatureswere werebelow below 100 100 °C. roll-lamination technique andand dipdip coating process soso the The procedure of of the thetransparent transparentflexible flexibleCMUT, CMUT,illustrated illustratedininFigure Figure is described The fabrication fabrication procedure 5, 5, is described as as follows: follows: 1. 1. 2.

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Paste aa 125 125 µm thick ITO-PET substrate onto a silicon wafer. Paste wafer. Spin coat a 2 µm thick SU-8 2002 photoresist onto the ITO-PETsubstrate, substrate, followed soft bake Spin µm thick SU-8 2002 photoresist onto the ITO-PET followed byby soft bake at ◦ ◦ ◦ at 3 min, 3 min, and then 3 min. 6565C°C forfor 3 min, 9595C°C forfor 3 min, and then 6565C°C forfor 3 min. Pattern sidewall. Perform post exposure exposure bake bake at at 65 65 ◦°C for Pattern the the SU-8 SU-8 2002 2002 photoresist photoresist to to form form aa sidewall. Perform aa post C for ◦ ◦ ◦ ◦ 33 min, min, 95 95 °CC for for 33 min, min,and and65 65°CCfor for33min, min,and andthen thenhard hardbake bakeatat6565°CCfor for2 2min, min,9595°CCfor for4 min, and 6565 °C◦ C forfor 2 min. 4 min, and 2 min. Prepare µm thick thickSU-8 SU-82002 2002photoresist photoresiston ona aPET PET release layer, followed bake at ◦65 Prepare aa 44 µm release layer, followed by by softsoft bake at 65 C °C 3 min then 2 min. forfor 3 min andand then 95 ◦95 C °C forfor 2 min.

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Roll-laminate the PET release layer containing the SU-8 2002 photoresist as vibrating membranes Roll-laminate theatPET containing theMPa. SU-8 2002 photoresist as vibrating membranes onto the sidewall an release averagelayer pressure of 0.35 onto the sidewall at an average pressure of 0.35 MPa. Expose the SU-8 2002 photoresist on the PET release layer, followed by post exposure bake at ◦ C for the ◦ C for SU-8 photoresist on 65 the◦PET followed by post exposure bakedevelop at 65 65Expose 1 min, 952002 1 min, and C forrelease 1 min.layer, Remove the PET release layer and °C for 1 min, 95 °C for 1 min, and 65 °C for 1 min. Remove the PET release layer and develop the the vibrating membranes. vibrating membranes. Prepare a 0.2 µm thick transparent silver nanowire electrode through dip coating. Prepare a 0.2 µm thick transparent silver nanowire electrode through dip coating. Spin coat a 1 µm thick SU-8 2002 photoresist onto the vibrating membranes, followed by soft bake Spin coat a 1 µm thick SU-8 2002 photoresist onto the vibrating membranes, followed by soft at 65 ◦ C for 2 min, 95 ◦ C for 2 min, and then 65 ◦ C for 2 min. Pattern the SU-8 2002 photoresist to bake at 65 °C for 2 min, 95 °C for 2 min, and then 65 °C for 2 min. Pattern the SU-8 2002 ◦ C for 2 min, and 65 ◦ C form a protecttolayer. post Perform exposurea bake at 65 ◦ C for 2 min, photoresist form Perform a protectalayer. post exposure bake at 6595 °C for 2 min, 95 °C for 2 ◦ ◦ ◦ formin, 2 min, hard 65 hard C forbake 2 min, 95°CCfor for2 3min, min, 65 3 C for and 2 min. andand 65 °C for bake 2 min,atand at 65 95and °C for min, 65 °C for 2 min. Remove CMUTfabrication. fabrication. Removethe thesilicon siliconwafer waferto tocomplete complete the the transparent transparent CMUT

Theminimum minimummembrane membrane thicknesses thicknesses for for sacrificial-layer sacrificial-layer and The and roll-lamination roll-laminationfabrication fabrication proceduresare are1 1µm µm and and 2 µm, limitation of membrane thickness in the in roll-the procedures µm, respectively. respectively.TheThe limitation of membrane thickness lamination fabrication procedure is due to the removal of the PET release layer in step 6. Considering roll-lamination fabrication procedure is due to the removal of the PET release layer in step 6. the ratio ofthe membrane thickness over diameter, sacrificial-layer fabrication fabrication procedure achieves Considering ratio of membrane thickness overthe diameter, the sacrificial-layer procedure the state-of-the-art in micromachining polymer-based CMUTs. achieves the state-of-the-art in micromachining polymer-based CMUTs.

Figure fabricationtechnique. technique. Figure4.4.CMUT CMUTmade madeby by the the sacrificial-layer sacrificial-layer fabrication

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Figure Figure 5. 5. Transparent Transparent CMUT CMUT made made by by the the roll-lamination roll-lamination fabrication fabricationtechnique. technique.

Figure 6a,b 6a,b illustrate illustrate two two CMUTs CMUTs made made respectively respectively by by the the sacrificial-layer sacrificial-layer and and roll-lamination roll-lamination Figure techniques. Both figures show top electrodes made of gold because the electrode patterns areclear not techniques. Both figures show top electrodes made of gold because the electrode patterns are not clearsilver with silver nanowire. Theand leftright and right images obtained an optical microscope with nanowire. The left images were were obtained usingusing an optical microscope (OM)(OM) and and scanning electron microscope (SEM). The etching holes from the sacrificial-layer technique are scanning electron microscope (SEM). The etching holes from the sacrificial-layer technique are clearly clearly shown in Figure 6a, whereas there was no hole seen through the roll-lamination technique in shown in Figure 6a, whereas there was no hole seen through the roll-lamination technique in Figure 6b. Figure 6b. A transparent flexible CMUT was successfully fabricated using the proposed roll-lamination A transparent CMUT was successfully fabricated the proposed roll-lamination techniques. Figure flexible 7 presents non-transparent (left image) andusing transparent (right image) CMUTs techniques. Figure 7 presents image) and atransparent (right image) CMUTs with with gold and silver nanowirenon-transparent top electrodes. (left Figure 8 shows transparent CMUT under deflection. gold and silver nanowire top electrodes. Figure 8 shows a transparent CMUT under deflection. The The performance characteristics of these two CMUTs were tested and compared. performance characteristics of these two CMUTs were tested and compared.

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(a) (b) (a) (b) (a) (b) Figure 6. (a) CMUT made by the sacrificial-layer technique (left: optical microscope (OM) image; Figure 6.6.(a) made by technique (left: Figure (a)CMUT CMUT made by the sacrificial-layer technique (left: opticalmicroscope microscope(OM) (OM)image; image; Figure (a) CMUT made bythe thesacrificial-layer sacrificial-layer technique (left:optical optical microscope (OM) image; right:6.scanning electron microscope (SEM) image); (b) CMUT made by the roll-lamination technique right: scanning electron microscope (SEM) image); (b) CMUT made by the roll-lamination technique right: scanning electron microscope (SEM) image); (b) CMUT made by the roll-lamination technique right: electron (left:scanning OM image; right: microscope SEM image).(SEM) image); (b) CMUT made by the roll-lamination technique (left: (left: OM image; right: SEM image). (left:OM OMimage; image;right: right:SEM SEMimage). image).

Figure 7. CMUTs with gold (left) and silver nanowire (right) top electrodes. Figure Figure 7.CMUTs CMUTswith with gold (left) and silver nanowire (right) top electrodes. Figure7.7. CMUTs withgold gold(left) (left)and andsilver silvernanowire nanowire(right) (right)top topelectrodes. electrodes.

Figure 8. A transparent flexible CMUT.

4. Discussion

Figure 8.8.AAtransparent flexible CMUT. Figure CMUT. Figure 8. A transparent transparent flexible flexible CMUT.

4.4.Discussion 4.1. Roll-Lamination Fabrication 4. Discussion Discussion Fabricating a flexible CMUT through roll-lamination techniques involves two critical steps: (1) 4.1. Roll-Lamination Fabrication Roll-Lamination 4.1.preparing Fabrication vibrating membranes on a PET release layer, and (2) laminating the membranes onto the Fabricating a flexible CMUT through roll-lamination techniques involves two critical (1) sidewall. The SU-8 vibrating membranes be prepared on a PET release layer with asteps: baking Fabricating aa flexible CMUT through roll-lamination techniques involves twotwo critical steps: (1) flexible CMUT throughshould roll-lamination techniques involves critical steps: preparing vibrating membranes on a PET release layer, and (2) laminating the membranes onto the temperature below 100 °C. If the temperature is higher than 120 °C, membrane deformation occurs preparing vibrating membranes onon a PET release (1) preparing vibrating membranes a PET releaselayer, layer,and and(2) (2)laminating laminatingthe the membranes membranes onto the on the The PETSU-8 release layer. The membraneshould preparation in step (4) baking times increasing more sidewall. vibrating membranes be prepared onwith a PET release layer with a baking sidewall. The SU-8 vibrating vibrating membranes membranes should should be be prepared prepared on a PET release layer with a baking than 10 s result in100 an excessively dry SU-8 photoresist, which causes the membranes to fail to laminate temperature below °C. If the temperature is higher than 120 °C, membrane deformation occurs higher than than 120 120 ◦°C, membrane deformation deformation occurs temperature below 100 ◦°C. C. If the temperature is higher C, membrane ontoPET the release sidewall; this isThe evident when many bubblesin form at(4) the junction between sidewall more and on the layer. membrane preparation step baking timesthe increasing layer. The membrane preparation in step (4)with on the PET release layer. with baking times increasing more the membrane during lamination. SU-8 The baking times decreasing more than 10 s lead to to fail an overly wet than to laminate than10 10s sresult resultininan anexcessively excessivelydry dry SU-8photoresist, photoresist,which whichcauses causesthe themembranes membranes membranes to fail to laminate photoresist, which causes the membranes to detach from the sidewall when the release layer is onto the sidewall; this is evident when many bubbles form at the junction between the sidewall and onto the sidewall; this is evident when many bubbles form atthere the junction between the sidewall and removed; this is evident when the cavity is exposed or when are holes on the membranes. the membrane during lamination. The baking times decreasing more than 10 lead totoan overly wet during lamination. The times decreasing more than 10s s10 lead an wet the membrane membrane during lamination. Thebaking baking times decreasing more than s lead tooverly an overly Successful lamination of vibrating membranes onto the the sidewall sidewall depends largely on theis photoresist, which causes the membranes to detach from when the release layer photoresist, which causes the membranes to detach from the sidewall when the release layer wetlamination photoresist, whichWhen causesthe the membranes to detach from0.3 theMPa, sidewall when the release layer is pressure. lamination pressure is below the membranes and sidewall removed; this is evident when the cavity is exposed or when there are holes on the membranes. evident when the the cavity cavity is exposed exposed or MPa, whenthe there are holes holes onthe the membranes. removed; this iswhen evident when is or when there are on membranes. fail to bond; the lamination pressure is over 0.6 sidewall becomes deformed. A Fuji Successful lamination ofof vibrating membranes onto the sidewall depends largely on the Successful lamination vibrating membranes onto the sidewall depends largely on Successful lamination of vibrating membranes onto the sidewall depends largely on the lamination Prescale film is used to measure the lamination pressure and its uniformity on the membranes; thisthe lamination pressure. When the lamination pressure is below 0.3 MPa, the membranes and sidewall lamination pressure. When the pressure lamination pressure below MPa, the and membranes pressure. When the lamination is below 0.3 is MPa, the0.3 membranes sidewalland fail sidewall to bond; fail to bond; when the lamination pressure is over 0.6 MPa, the sidewall becomes deformed. fail to bond; when the lamination pressure is over 0.6 MPa, the sidewall becomes deformed.AAFuji Fuji Prescale Prescalefilm filmisisused usedtotomeasure measurethe thelamination laminationpressure pressureand andits itsuniformity uniformityon onthe themembranes; membranes;this this

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when the lamination pressure is over 0.6 MPa, the sidewall becomes deformed. A Fuji Prescale film is Sensors 2017, 17, 1443 7 of 16 used to measure the lamination pressure and its uniformity on the membranes; this is achieved by placing the film by on placing the PET release observing the color change thechange releaseonlayer following is achieved the film onlayer, the PET release layer, observing theon color the release lamination (yellow:lamination >0.6 MPa; red: 0.3–0.6 MPa; green:MPa; 0.6 MPa; red: 0.3–0.6

Development of a Novel Transparent Flexible Capacitive Micromachined Ultrasonic Transducer.

This paper presents the world's first transparent flexible capacitive micromachined ultrasonic transducer (CMUT) that was fabricated through a roll-la...
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